The resistance of the photoresistor. Dark resistance Rсo value depends on the shape, size, temperature, and physical and chemical nature of the photosensitive layer of photoresist. Very high dark resistance (from 10 ^ 4 to 10 ^ 9 ohms at 25 ° C) have PbS, CdS, CdSe. Small (10 to 10 3 ohms at 25 ° C), dark resistance in InSb, InAs, CdHgTe. The resistance of the photoresistor exposed, decreases rapidly with increasing exposure (Fig. 2).

Properties can be described photoresistor equivalent electrical circuit in which dark resistance Rso are connected in parallel with the resistance Rcp, whose value is determined by the photoelectric effect of the incident light intensity:

Rcp = аФ ^-γ

where a depends on the material, temperature, and the spectrum of the incident light, and γ typically has a value of 0.5 to 1. Under these conditions, the total resistance Rc is given by